1. Field of the Invention
The present invention relates to digital communicating method and apparatus and, more particularly, to digital communicating method and apparatus for modulating a digital signal into a chirp signal and communicating.
2. Related Background Art
Hitherto, as a method of modulating a digital signal and transmitting, there has widely been used a frequency shift keying modulation system (FSK) in which two different frequencies are prepared and are transmitted in a form of a frequency change by making them correspond to "1" and "0" of the binary digital signal or a phase shift keying modulation system (PSK) in which "1" and "0" of the digital signal are made correspond to a phase change of a carrier, or the like.
The FSK is the modulation system in which frequencies f.sub.1 and f.sub.2 of the carrier are assigned to "1" and "0" of the digital signal, respectively, and on the reception side, "1" and "0" of the signal are discriminated by detecting the frequency of the signal. On the other hand, the PSK is the modulation system in which phases .phi..sub.1 and .phi..sub.2 of the carrier are assigned to "1" and "0" of the digital signal, and on the reception side, the phase of the signal is detected.
Although the quality of the digital communication is generally evaluated by a code error rate, in the above modulation systems, the code error rate depends on the ratio (C/N ratio) of an electric power of the carrier of the signal to the noises. As the C/N ratio decreases when the signal becomes very weak, the code error rate promptly deteriorates. Therefore, in the conventional modulation systems such as FSK, PSK, and the like, in order to guarantee the necessary code error rate, it is necessary to keep the C/N ratio of a predetermined value or more. Thus, there is a drawback such that the adaptive range of the transmission is limited by such a necessity.
On the other hand, hitherto, the optical spatial communication for performing the communication by using the atmospheric propagation of the optical signal has been known. In the optical spatial communication, since it is easily influenced by the characteristic disturbance of the transmission path called an open space, it is a general way that the signal is subjected to an arbitrary modulation and is transmitted rather than it is directly transmitted in the original form.
In the case of the optical spatial communication, when the transmission of a long distance is executed in the open air or the like, the signal light is largely attenuated in dependence on the weather conditions such as rainfall or the like. Therefore, it is necessary to detect a very weak signal. In the conventional modulation systems such as FSK, PSK, and the like, the communication distance is limited and there is also a problem of the reliability.
On the other hand, as an optical spatial communication of a short distance, it is considered to diffuse the signal light to enlarge the space range where the signal can be received instead of the communication of the one-to-one corresponding relation. However, even in such a case, since the signal light level becomes very small, there is a drawback such that it is also similarly difficult to keep the communication reliability according to the conventional transmission systems.
On the other hand, hitherto, a system such as frequency multiplex, time-sharing multiplex, wavelength multiplex, or the like has been used as a system for multiplexing digital signals and transmitting.
The frequency multiplex system relates to a method whereby signals which were modulated with an occupation band width are arranged and multiplexed on the frequency base at a frequency interval such as not to overlap the signals. On the other hand, the timesharing multiplex system relates to a method whereby a plurality of digital signals which are input by the digital transmission are multiplexed as a pulse train in which they are arranged on the time base at a predetermined time interval.
On the other hand, the wavelength multiplex system relates to a method whereby it is executed by the optical communication, information is added to lights of different wavelengths, the lights are transmitted by the same transmission path, and thereby multiplexing.
A proper one of the above various kinds of multiplex transmission systems is selected in accordance with the content, specifications, and the like of a system which is constructed. However, the above conventional multiplex transmission systems need the advanced techniques in the signal multiplex and the separation of the multiplexed signal and its construction is also complicated. There are also problems such that the multiplexing apparatus and the demultiplexing apparatus are large in size and expensive.